Engine control systems with knock detection capability, are used to detect and eliminate knocking conditions, characteristic of the operation of internal combustion engines. Eliminating a knocking condition is important because, left unchecked engine power and efficiency will suffer, and combustion chamber and spark plugs will be damaged.
Knock detection systems typically use an accelerometer based sensor mounted on an engine for converting engine vibration into an electrical signal. This electrical signal includes a broad spectra of information about the engine's operating condition. Included in this signal is the knock information, and extraneous information known as noise. When knocking occurs, the knock component of the signal results from gas oscillations in the combustion chamber during combustion. The noise component of the signal generally is significant and can be large enough to mask the knock component. The noise component may be comprised of several sources including piston slap vibration, valve dosing vibration, and other systemic noise.
Various techniques have been applied to extract the knock information from the electrical signal representative of engine vibration, however, these techniques have been limited in accuracy and reliability. The knock component of the signal varies considerably in magnitude and frequency over the full operating range of the engine. This partially is related to, gas temperature, and combustion chamber geometry. Moreover, there are variations from engine to engine and from cylinder to cylinder within the same engine that affect the relationship of the knock component to the noise component.
To provide for this range of conditions, typically knock detection schemes use a broadband bandpass filter to extract knock information from the broadband electrical signal. Some schemes include engine crank angle based windowing, to restrict the analysis of knock to when it could likely occur in the combustion cycle. Other schemes attempt to improve the signal-to-noise ratio of the measurement by employing a second, or noise, bandpass filter having a passband located distant to the passband of the knock bandpass filter. The output of this noise bandpass filter is then subtracted from the output of the knock bandpass filter. The concept here is that the noise bandpass filter will indicate the level of noise signal located apart from the knock bandpass filter. This assumes that noise is broadband, and that the noise component in the knock band is of substantially the same magnitude. Thus, by subtracting the output of the noise bandpass filter from the output of the knock bandpass filter a more accurate representation of the knock component may be obtained.
This scheme is defective for several reasons, including that the filters intrinsically have a phase delay error. When the noise is subtracted this error detracts from accurately canceling the noise component of the signal. Further, since the noise band is located apart, only the noise located apart from the characteristic knock frequencies can be detected. In fact, the noise is not really broadband and there can be substantial noise located close to the characteristic knock frequencies that will not be canceled in this scheme. Also, with present schemes, extensive calibration is required to determine for each engine time a characteristic knock frequency, and if used, appropriate crank angle window, and a knock threshold. Additionally, as the engine speed increases, the systemic noise content increases substantially, and current systems perform inadequately. Finally, in these schemes the knock detection threshold is not adjusted with sensor gains and system aging.
To further improve accuracy some systems use a multi-cycle averaging technique. Because of the dynamic characteristic of combustion within the combustion chamber, the knock component has a random behavior and therefore multi-cycle averaging works poorly.
What is needed is an improved system for detecting knock in internal combustion engines that is more reliable, and accurate, can detect knock at high engine speeds, requires minimum calibration and can be easily applied to different engine families.